Photo-sensor, measurement apparatus and camera system

ABSTRACT

The present invention provides a photo-sensor with a stable current limiting function and pixel reset function. When the incident light quantity of the phototransistor is equal to or less than a predetermined quantity and the base potential of the phototransistor is in a first potential of an operation point in a stationary state, an MOSFET for discharging an electric charge is controlled so as to be turned OFF. In addition, when the incident light quantity of the phototransistor is equal to or more than the predetermined quantity, a MOSFET for detecting an electric current is controlled so as to operate in a saturation region. When the base potential of the phototransistor has changed to a second potential from the first potential, the MOSFET for discharging an electric charge is controlled so as to be turned ON.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photo-sensor which converts a photosignal to an electric signal and amplifies the electric signal with aphototransistor, and to a measurement apparatus and a camera systemusing the same.

2. Description of the Related Art

A photo-sensor for photometry (hereinafter referred to as “AE sensor”)which is used in a digital single-lens reflex camera or the like needsto have a wide dynamic range performance that corresponds to a rangefrom a low luminance to a high luminance and have a responsivity offollowing the change of incident light. As for the dynamic range, aphoto-sensor is used which amplifies a photo-current with aphototransistor so as to enhance the measurement accuracy in a lowluminance condition, during which a signal to be generated is small.

Such a photo-sensor has a problem that an electric current consumptionincreases in a high luminance period. For this reason, a method oflimiting the electric current consumption of the photo-sensor to a fixedvalue is employed (hereinafter referred to as “current limiting”) Forinstance, a system of controlling a portion discharging excessivecarrier, based on a signal quantity which has been output from aphotoelectric conversion element is employed in a photoelectricconversion apparatus described in Japanese Patent Application Laid-OpenNo. 2000-244004.

On the other hand, as for the responsivity with respect to the incidentlight, for instance, a system of varying only an emitter potentialwithout varying the base potential of the phototransistor is employed asis described in Japanese Patent Application Laid-Open No. 2000-77644.Furthermore, FIG. 5 in Japanese Patent Application Laid-Open No.2000-77644 discloses a circuit configuration for performing the currentlimiting.

In the photoelectric conversion apparatus of FIG. 1 in Japanese PatentApplication Laid-Open No. 2000-244004, when an electric current whichhas been amplified by the phototransistor exceeds a limit current, agate potential of a MOSFET for discharging an electric charge abruptlyincreases due to a MOSFET for detecting an electric current. Then, theMOSFET for discharging an electric charge is turned ON, and thereby anexcessive photo-current is discharged through the base of thephototransistor.

In other words, such a feedback circuit is formed that when a basecurrent of the phototransistor changes, a drain voltage of the MOSFETfor detecting an electric current changes, and a drain current of theMOSFET for discharging an electric charge changes. Then the base currentof the phototransistor changes and the base potential of thephototransistor changes.

In this configuration, the MOSFET for discharging an electric chargecontrols the base of the phototransistor through a configuration ofgrounding the source, so once the MOSFET has been turned ON, the MOSFETcontinues to excessively discharge the photo-current until the gatepotential changes by the feedback. A similar feedback occurs when theMOSFET for discharging an electric charge is turned OFF. The decrease ofthe stability due to a phase delay of this feedback can cause anoscillation state of making the MOSFET for discharging an electriccharge switch repeatedly between ON and OFF.

When the MOSFET for discharging an electric charge serves as a resetswitching MOSFET for an operation of resetting a pixel, which will bedescribed later, the gate potential of the MOSFET cannot be directlycontrolled, so a complicated control becomes necessary. Furthermore,there has been a problem that the pixel cannot be reset in someoperation conditions.

Also in a circuit which is shown in FIG. 5 of Japanese PatentApplication Laid-Open No. 2000-77644, when a base current of thephototransistor changes, a drain voltage of the MOSFET for detecting anelectric current changes. Then a drain current of the MOSFET fordischarging an electric charge changes, and the base current of thephototransistor changes and the base potential of the phototransistorchanges. Accordingly, the circuit of FIG. 5 has the same problem as thatof the circuit shown in FIG. 1 of Japanese Patent Application Laid-OpenNo. 2000-244004.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a photo-sensor, ameasurement apparatus, and a camera system which amplify a photo signalwith a phototransistor and output the photo signal, which can realize astable current limiting function and pixel reset function photo-sensor.

A photo-sensor according to the present invention includes: aphototransistor for receiving light to generate a photo-current and tooutput a current produced by amplifying the photo-current; a firstMOSFET for detecting the current from the phototransistor; and a secondMOSFET having a one non-controlling terminal connected to a base of thephototransistor, to discharge an excessive photo-current in thephototransistor, wherein the second MOSFET is turned OFF when a lightquantity incident in the phototransistor is smaller than a predeterminedvalue and a base potential of the phototransistor is at a firstpotential, and the first MOSFET operates in a saturation region under acondition that the incident light quantity of the phototransistor islarger than a predetermined value, and a gate potential of the secondMOSFET is fixed so that the second MOSFET is turned ON, when the basepotential of the phototransistor changes from the first potential to asecond potential during a sensor operation period for outputting thephoto-current from the phototransistor.

The photo-sensor according to the present invention controls a gatepotential of a MOSFET for discharging an electric charge so that theMOSFET for discharging an electric charge is turned ON, when a basepotential of the phototransistor has deviated from an operation point inthe stationary state by a certain voltage. Therefore, the photo-sensorcan eliminate the delay of the phase of the feedback in a currentlimiting operation and can enhance its stability in the current limitingoperation.

Furthermore, the photo-sensor sets a drain potential of the MOSFET fordetecting an electric current at an appropriate potential for anoperation of resetting the pixel, and thereby can conduct a stablepixel-resetting operation in such a circuit which makes the basepotential unchanged as is described in Japanese Patent ApplicationLaid-Open No. 2000-77644 and the like. As a result, a camera system canbe realized which mounts an AE sensor thereon that is provided with astable current limiting function and a pixel reset function withoutincreasing the cost.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a photo-sensor according toEmbodiment 1 in the present invention.

FIG. 2 is a circuit diagram illustrating a photo-sensor according toEmbodiment 1 in the present invention.

FIG. 3 is a circuit diagram to be employed when a photo-sensor of thepresent invention is used as an AE sensor.

FIGS. 4A, 4B, 4C and 4D are views illustrating an operation timing ofEmbodiment 1 in the present invention.

FIG. 5 is a circuit diagram illustrating one example of a circuit whichgenerates a middle potential, in the photo-sensor of FIG. 3.

FIG. 6 is a block diagram illustrating one embodiment of a lightmeasuring apparatus with the use of a photo-sensor according to thepresent invention.

FIG. 7 is a block diagram illustrating one embodiment of a camera systemwith a light measuring apparatus according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

Embodiment 1

FIG. 1, FIG. 2 and FIG. 3 are circuit diagrams illustrating aphoto-sensor according to Embodiment 1 in the present invention. FIG. 1is a circuit diagram best illustrating a feature of the presentinvention. FIG. 2 is a circuit diagram in which the polarity of a MOSFETfor discharging an electric charge in FIG. 1 is changed. FIG. 3 is acircuit diagram illustrating one embodiment preferable for the casewhere the photo-sensor is applied as an AE sensor (light measuringapparatus which is used in a camera and the like) by adding a circuitwhich makes the base potential of a phototransistor unchanged to thecircuit of the photo-sensor in FIG. 1.

In these figures, a phototransistor 101 is a bipolar type. Thephototransistor 101 receives a light in a base portion, and outputs anelectric current of an amplified photo-current, through an emitter. AMOSFET 102 for discharging an electric charge discharges an excessivephoto-current of the phototransistor 101, and a MOSFET 106 for detectingan electric current detects an emitter current of the phototransistor101. The MOSFET 106 for detecting an electric current is referred to asa first MOSFET, and the MOSFET 102 for discharging an electric charge isreferred to as a second MOSFET.

A base terminal 103 of the phototransistor 101, a gate terminal 104 ofthe MOSFET 102 for discharging an electric charge, a drain terminal 105of the MOSFET 102 for discharging an electric charge and a gate terminal107 of the MOSFET 106 for detecting an electric current are also shownin the drawings. An NPN type is used for the phototransistor 101, but aPNP type may be used instead.

In the following description, the base potential of the base terminal103 of the phototransistor 101 is represented by Vbase, the gatepotential of the gate terminal 104 of the MOSFET 102 for discharging anelectric charge is represented by Vgate, and the drain potential of thedrain terminal 105 of the MOSFET 102 for discharging an electric chargeis represented by Vdrain.

In FIG. 1, a drain terminal which is one non-controlling terminal of aMOSFET 106 for detecting an electric current, or a first MOSFET, isconnected to an emitter of an output terminal of a phototransistor 101,and works in a saturation region when the current value reaches apredetermined current value. A source terminal which is onenon-controlling terminal of a MOSFET 102 for discharging an electriccharge, or a second MOSFET is connected to a base terminal 103 of thephototransistor 101. A predetermined potential is applied to a drainterminal 105 that is the other non-controlling terminal of the secondMOSFET 102, which will be described later.

In FIG. 2, a source terminal that is one non-controlling terminal of theMOSFET 102 for discharging an electric charge, which is the secondMOSFET, and a gate terminal of the MOSFET 102 are connected to the baseterminal 103 of the phototransistor 101. A predetermined potential isapplied to a drain terminal 105 that is the other non-controllingterminal of the second MOSFET 102, as will be described later. Thecircuit in FIG. 2 can make the polarities of the MOSFET 106 fordetecting an electric current and the MOSFET 102 for discharging anelectric charge the same, unlike in FIG. 1. As a result, elements do notneed to be separated by a diffusion well, therefore the layout area canbe reduced.

Furthermore, in FIG. 3, a current source 201 drives a feedback circuitfor making a base potential of the phototransistor 101 unchanged. AMOSFET 202 detects the variation of the base potential of thephototransistor 101 and a MOSFET 203 controls an emitter potential ofthe phototransistor 101.

Subsequently, the operation will now be described. When a light isincident on a photo-sensor, a photo-current corresponding to lightintensity is generated through photoelectric conversion. The generatedphoto-current flows into a base terminal 103 of a phototransistor 101,and is amplified to be an emitter current. The phototransistor 101 isprovided with a feedback circuit as is illustrated in FIG. 3, so as toenhance response characteristics when the light intensity suddenlychanges. That is to say, a base potential Vbase of the phototransistor101 is controlled so as not to deviate from a predetermined operationpoint (hereinafter referred to as “Vb potential”). In the operation inFIG. 3, the description of a part which overlaps with Japanese PatentApplication Laid-Open No. 2000-77644 will be omitted.

Specifically, a gate voltage Vgate is controlled so that when the basepotential Vbase of the phototransistor 101 rises to a potential betweena potential Vb of a predetermined operation point and a power sourcevoltage V (hereinafter referred to as “Vlimit potential”), the MOSFET102 for discharging an electric charge is turned ON. In a MOSFET 106 fordetecting an electric current, which is a first MOSFET, a potential of asource terminal is fixed, and a gate potential is given to a gateterminal 107 so that the MOSFET 106 operates in a linear region innormal condition, but operates in a saturation region when a draincurrent equal to or more than the limit current value flowstherethrough.

Specific numeric values will be shown below. For instance, a powersource voltage V is 5.0 V, a Vb potential is 3.5 V, a Vlimit potentialis 4.5 V, a Vgate potential is 3.3 V, a Vdrain potential is 3.6 V, and athreshold voltage of a PMOS is 1.2 V (in consideration of substrate biaseffect). In addition, a drain current flowing when the MOSFET 106 fordetecting an electric current operates in the saturation region is 200μA, and the HFE (current amplification factor) of the phototransistor101 is 100.

Subsequently, a reason why the MOSFET 102 for discharging an electriccharge, which is a second MOSFET, is necessary will now be described. Atfirst, when intense light is incident on the photo-sensor and theemitter current of the phototransistor 101 increases to the limitcurrent value or more, an operation region of the MOSFET 106 fordetecting an electric current, which is a first MOSFET, shifts to asaturation region from a linear region. Then, the emitter potential ofthe phototransistor 101 rises to the vicinity of the power sourcevoltage.

At this time, when there is no MOSFET 102 for discharging an electriccharge, the phototransistor 101 operates to pass an emitter currentequal to or more than a limit current value therein, causing a Vbasepotential of the base terminal 103 to rise above the power sourcevoltage. When the Vbase potential exceeds the power source voltage, acollector and a base of the phototransistor 101 are forward-biased,which is originally used in a reverse bias, causing a photo-current toflow to a collector from a base.

The phototransistor 101 normally receives light in a region between thecollector and the base. When considering the case where a collector iscommonized between adjacent phototransistors, a parasitic bipolartransistor is formed in between the bases of the adjacentphototransistors. When the collector and the base are forward-biased andthe current flows to the collector from the base, as was describedabove, the parasitic bipolar transistor operates, and the current flowsto the base of the adjacent phototransistor. As a result, there is aconcern that a crosstalk of photo signals occurs between adjacentpixels.

For this reason, the MOSFET 102 for discharging an electric charge,which is a second MOSFET, is employed so as to clip the Vbase potentialof the phototransistor 101 at a Vlimit potential lower than the powersource voltage, and discharge an excessive photo-current as a draincurrent of the MOSFET 102 for discharging an electric charge. As aresult, the increase of the emitter current of the phototransistor 101becomes saturated at the limit current value, and the MOSFET 102 canrealize a current limiting function.

A difference between the present circuit and a circuit in FIG. 1 ofJapanese Patent Application Laid-Open No. 2000-244004 exists in a pointthat the stability of the present circuit is improved in a period untilthe photo-current settles in a stationary state in which currentlimiting operates after an increase of a photo-current followed by thestart of discharging an excessive carrier by the turn-on of the MOSFET102 for discharging an electric charge. The circuit in the presentembodiment has the MOSFET 102 which clips the base potential of aphototransistor 101 arranged therein, in place of a source grounded MOStransistor 7 which is used for discharging an electric charge in thecircuit of FIG. 1 in Japanese Patent Application Laid-Open No.2000-244004.

The circuit of FIG. 1 in Japanese Patent Application Laid-Open No.2000-244004 can cause an oscillation state in which the MOSFET fordischarging an electric charge repeats switching between ON and OFF dueto a delay of feedback, as was described above. In the circuit of thepresent embodiment, such a feedback is not formed as is formed in thecircuit of FIG. 1 in Japanese Patent Application Laid-Open No.2000-244004, so the delay of the feedback does not occur in a periodafter the Vbase potential changes until the drain current of the MOSFET102 for discharging an electric charge changes. As a result, the concernis removed that the circuit may cause the oscillation state, so thestability of the current limiting operation is improved.

When the photo-sensor according to the present invention is used as anAE sensor, it is necessary to reset a base potential Vbase of thephototransistor 101 to a predetermined voltage (pixel-resettingoperation) before its operation, in order to quickly settle the Vbasepotential into a stationary state. In order to conduct thepixel-resetting operation, the photo-sensor needs to have a switchelement such as a MOSFET provided therein. If a switching MOSFET forresetting a pixel was separately provided therein, the switching MOSFETwould cause a problem of lowering the aperture ratio of the AE sensor.Accordingly, it is desirable that the MOSFET 102 for discharging anelectric charge also serves as the switching MOSFET.

Because the circuit of FIG. 1 in Japanese Patent Application Laid-OpenNo. 2000-244004 cannot directly control the gate potential Vgate of aMOSFET for discharging excessive carrier, a complicated control would berequired to make the MOSFET 102 for discharging an electric charge toalso serve as the resetting MOSFET,. In that case, the control method isconsidered, for instance, which includes turning the MOSFET 106 fordetecting an electric current OFF to turn the gate potential Vgate ofthe MOSFET 102 for discharging an electric charge High.

Furthermore, the circuit configuration of FIG. 1 in Japanese PatentApplication Laid-Open No. 2000-244004 has a problem in an operationrange. In the circuit of the photo-sensor illustrated in FIG. 3, thebase potential Vbase of the phototransistor 101 shows a potential lowerthan the power source voltage by a threshold voltage and overdrivevoltage of a PMOS, and accordingly is set at a high potential which iscomparatively close to the power source voltage. In the circuit of FIG.1 in Japanese Patent Application Laid-Open No. 2000-244004, an NMOS isused in the MOSFET for discharging an electric charge so it is difficultto set the base potential Vbase to a high voltage. Furthermore, thethreshold value of an MOSFET increases in a high-temperature state, so avoltage range in which an electric charge can be injected into the basepotential Vbase becomes even narrower.

The circuit in the present embodiment directly controls the gatepotential Vgate of a gate terminal 104 of the MOSFET 102 for dischargingan electric charge, which is a second MOSFET, and therefore can easilymake the MOSFET 102 for discharging an electric charge also serve as theresetting MOSFET. In addition, the circuit can use a PMOS as the MOSFET102 for discharging an electric charge, and the base potential Vbase canaccordingly be set at a high potential regardless of the threshold valueof the MOSFET.

FIGS. 4A to 4D are views illustrating an operation timing of thephoto-sensor in FIG. 3. FIGS. 4A to 4D illustrate an operation timing inthe case where the photo-sensor performs a pixel-resetting operation aswas described above. FIG. 4A illustrates an operation timing in the caseof a low luminance (a case where the incident light quantity is equal toor less than predetermined quantity) including a pixel reset period.FIG. 4B illustrates an operation timing in the case of conducting acurrent limiting operation at a high luminance (a case where theincident light quantity is equal to or more than predetermined quantity)including a pixel reset period.

FIG. 4C illustrates an operation timing in the case where the incidentlight quantity changes from a low luminance to a high luminance and to alow luminance again after the photo-sensor performs the pixel-resettingoperation and settles in a stationary state. FIG. 4D illustrates thechange of a gate potential Vgate of a gate terminal 104 in a MOSFET 102for discharging an electric charge.

The abscissa axes of FIG. 4A to FIG. 4D represent an elapsed time (t).The ordinate axes of FIG. 4A to FIG. 4C show a Vbase potential of aphototransistor 101. The ordinate axis of FIG. 4D shows a gate potentialVgate of a MOSFET 102 for discharging an electric charge. In addition,in FIGS. 4A to 4D, a pixel reset period T1, a sensor operation period T2in a low luminance state, and a sensor operation period T3 in a currentlimiting state are shown.

A potential 307 in FIG. 4A to FIG. 4C is a Vbase potential of aphototransistor 101 when the MOSFET 102 for discharging an electriccharge is turned ON, in other words, is a Vlimit potential (secondpotential). A potential 308 is a Vdrain potential of the MOSFET 102 fordischarging an electric charge. A potential 309 is an operation point ofthe Vbase potential of the phototransistor 101 (hereinafter referred toas “Vb potential”) in an operation period in which the MOSFET 102 fordischarging an electric charge is not turned ON (first potential). Apotential 310 of FIG. 4D shows a set potential of a Vgate potential ofthe MOSFET 102 for discharging an electric charge in a sensor operationperiod.

A pixel-resetting operation will now be described with reference toFIGS. 4A to 4D. After a power source is turned on and when the basepotential Vbase of the phototransistor 101 is lower than that of the Vbpotential 309 which is a potential of a predetermined operation point,because the photo-sensor has to raise the base potential to the Vbpotential 309 with an infinitesimal photo-current, the rate of risebecomes very slow. Then, the photo-sensor injects an electric chargeinto a base terminal 103 of the phototransistor 101 to forcibly raisethe Vbase potential of the phototransistor 101 to a higher potentialthan the Vb potential 309, and then stops the injection.

By the above operation, the electric charge is immediately dischargedfrom the base of the phototransistor 101, and the base potential Vbaseof the phototransistor 101 is lowered. It is much faster to dischargethe electric charge by using a base current than to inject the electriccharge into the base of the phototransistor 101 by using aninfinitesimal photo-current. Accordingly, the circuit of FIG. 3 can bequickly settled at a stationary state by providing a pixel reset periodT1 as illustrated in FIGS. 4A to 4D.

A photo-sensor in the present embodiment makes a MOSFET 102 fordischarging an electric charge, which is a second MOSFET, also serve asa MOSFET for resetting a pixel, as was described above. The photo-sensorturns ON the MOSFET 102 for discharging an electric charge, which is thesecond MOSFET, in a pixel reset period T1, and sets the Vbase potentialof a phototransistor 101 at a Vdrain potential 308 of the MOSFET 102 fordischarging an electric charge, as are illustrated in FIGS. 4A to 4D.

The photo-sensor sets the Vgate potential of the MOSFET 102 fordischarging an electric charge at a potential 310 illustrated in FIG. 4Din a sensor operation period, and then settles the base potential Vbaseof the phototransistor 101 to a Vb potential 309 (FIG. 4A). Thepotential 310 shown in FIG. 4D is a potential at which the MOSFET 102for discharging an electric charge is turned ON, when the Vgatepotential rises to a potential (Vlimit potential) in between the Vbpotential and the power source voltage, as was described above.

In a period T3 in which the incident light quantity of the photo-sensoris equal to or larger than a predetermined quantity and increases, theMOSFET 102 for discharging an electric charge is turned ON when the basepotential Vbase of the phototransistor 101 has risen to a Vlimitpotential 307 (second potential). Therefore, an excessive charge isdischarged to a drain of the MOSFET 102 for discharging an electriccharge (see FIG. 4B and FIG. 4C).

After the current limiting operation, if the incident light quantityreaches a low luminance equal to or less than the predeterminedquantity, the Vbase potential of the phototransistor 101 settles at theVb potential 309 again (FIG. 4C). Here, the Vdrain potential 308 of theMOSFET 102 for discharging an electric charge is set at a potentialbetween the Vb potential 309 (first potential) and the Vlimit potential307 (second potential), as are illustrated in FIGS. 4A to 4D. By doingso, the MOSFET 102 for discharging an electric charge, which is thesecond MOSFET, can operate as a MOSFET for resetting a pixel as well.

A lateral overflow drain (hereinafter referred to as “LOD”) is generallyused for a photo-sensor which accumulates a photo-current in a parasiticcapacitance of a photodiode. When the pixel is reset with the use of anLOD in such a photo-sensor, the drain potential of the LOD is set at theinitial value of an accumulation potential.

In the present embodiment, a drain potential Vdrain 308 of a MOSFET 102for discharging an electric charge is set at a middle in between a Vbpotential 309 (first potential) and a Vlimit potential 307 (secondpotential), as was described above. As a result, the above method canrealize a stable operation of resetting a pixel when applied to aphoto-sensor which makes the base potential of the phototransistorunchanged, as is illustrated in the circuit of FIG. 3.

As was described above, when the incident light quantity of thephototransistor 101 is equal to or less than a predetermined quantityand the base potential of the phototransistor 101 is in the Vb potential309 (first potential), the MOSFET 102 for discharging an electriccharge, which is a second MOSFET, is turned OFF. In addition, when theincident light quantity of the phototransistor 101 is equal to or morethan the predetermined quantity, a MOSFET 106 for detecting an electriccurrent, which is a first MOSFET, operates in a saturation region (FIG.4C).

When the base potential of the phototransistor 101 changes to the Vlimitpotential 307 (second potential) from the Vb potential 309 (firstpotential), the MOSFET 102 for discharging an electric charge, which isthe second MOSFET, is turned ON (FIG. 4C). By turning on the secondMOSFET, an excessive photo signal is discharged.

FIG. 5 illustrates one example of a circuit which generates a gatepotential and a drain potential of a MOSFET 102 for discharging anelectric charge, an electric current of a current source 201 and thelike, in the photo-sensor of FIG. 3. FIG. 5 illustrates the circuit ofthe photo-sensor in FIG. 3, together. In the figure, a switch 401(SW_RESET) is provided for setting a gate potential Vgate of the MOSFET102 for discharging an electric charge at a GND potential in a pixelreset period T1. Terminals 104 and 105 in FIG. 5 are connected to a gateterminal 104 and a drain terminal 105 of the MOSFET 102 for dischargingan electric charge, respectively.

By controlling the switch 401, the gate 104 of the MOSFET 102 fordischarging an electric charge is controlled to the GND potential in thepixel reset period T1, and is controlled to a potential 310 illustratedin FIG. 4D in a sensor operation period.

A terminal (VCTRL) 402 controls a current value of a current source 201illustrated in FIG. 3. As is illustrated in FIG. 5, the current source201 includes a MOSFET, for instance, and the gate terminal receives asignal sent from the terminal 402. Therefore, a voltage between the gateand source of the MOSFET is determined, and the current quantity of thecurrent source 201 is determined.

In this way, a reference current which generates a gate voltage anddrain voltage of the MOSFET 102 for discharging an electric charge,which is a second MOSFET, is generated in the same circuit whichgenerates a reference current that generates a voltage for controllingthe base potential of a phototransistor 101.

The Vb potential 309, the Vgate potential 310 and the Vdrain potential308, which were described above, vary due to influences of processvariation and the like. When these parameters vary, a circuit operationcannot be correctly conducted. For instance, when the Vb potential 309rises, the MOSFET 102 for discharging an electric charge tends to beeasily turned ON, so that a photo-current leaks in a low luminanceperiod.

In addition, when the Vb potential 309 becomes higher than the Vdrainpotential 308, an electric charge cannot be correctly injected to a baseterminal 103 of the phototransistor 101, in the pixel reset period T1illustrated in FIGS. 4A to 4D. Then, the electric charge has to beinjected with an infinitesimal photo-current, so that a period of timeto be spent before the circuit of FIG. 3 settles in a stationary statebecomes long.

In the present embodiment, two reference currents are generated from theterminal (VCTRL) 402 for controlling the current source 201 whichdetermines the Vb potential 309, as is illustrated in FIG. 5. Thosereference currents generate the Vgate potential 310 and the Vdrainpotential 308, by being used as a drain current of the MOSFET which isconnected through a diode.

For instance, when the threshold value of a PMOS increases due to theprocess variation, the Vb potential 309 is lowered if the current of thecurrent source 201 does not change. On the other hand, the Vgatepotential 310 and the Vdrain potential 308 are similarly lowered,causing the Vgate potential 310 and the Vdrain potential 308 tointerlock with the variation of the Vb potential 310. A similar actionoccurs when the threshold value of the PMOS has decreased.

As a result, the circuit can interlock the process variation of theVgate and the Vdrain with the process variation of Vb, and accordinglycan realize a stable current limiting function against the processvariation. For information, in the circuit of FIG. 1, FIG. 2, FIG. 3,FIG. 5 and the like, the polarity of each element of the phototransistorand the MOSFET may be replaced with those of reverse polarity.Specifically, an NPN type of a phototransistor may be replaced with aPNP type of a phototransistor, and an NMOS in the MOSFET may be replacedwith a PMOS. Alternatively, a PMOS may be replaced with a NMOS.

Embodiment 2

FIG. 6 is a block diagram illustrating one embodiment where aphoto-sensor according to the present invention is used as an AE sensor.In the figure, an AE pixel 501, a logarithmically-compressingintegration circuit 502, a shift register 503 and an HFE compensationcircuit 504 are shown. A gain circuit 505, a circuit 506 which generatesan intermediate potential to be supplied to each block, and a band gapcircuit 507 are also shown.

Furthermore, a TG circuit 508 which generates a timing of each blockthrough communication with the outside, an AE signal output terminal509, a band gap voltage output terminal 510 and a logic IO terminal 511are shown.

The photo-sensor according to the present invention as described aboveis included in the AE pixel 501. However, a MOSFET 106 for detecting anelectric current is arranged in the AE pixel 501 or thelogarithmically-compressing integration circuit 502. The circuitillustrated in FIG. 5 is arranged in the circuit 506.

An HFE of the phototransistor in the logarithmically-compressingintegration signal which has been output from the shift register 503 iscanceled in the HFE compensation circuit 504, an appropriate gain isimposed to the signal in the gain circuit 505, and the resultant signalis output from the AE signal output terminal 509.

Embodiment 3

FIG. 7 is a block diagram illustrating one embodiment of a camera systemusing an AE sensor according to the present invention. In the figure, abarrier 601 protects a lens which will be described below and alsoserves as a main switch, a lens 602 focuses an optical image of anobject on a solid-state imaging device, and an aperture 603 adjusts thelight quantity which has passed through the lens 602. A solid-stateimaging apparatus 604 derives an image signal of the object which hasbeen focused through the lens 602, and an AE sensor (light measuringapparatus) 605 uses the photo-sensor according to the present invention,which was described in FIG. 6.

An imaged-signal processing apparatus 606 processes a signal which isoutput from the solid-state imaging device or a focus detectingapparatus, and an A/D converter 607 converts the signal which has beenoutput from an imaged-signal processing circuit from analog to digitalform. A signal processing section 608 subjects an image data which hasbeen output from the A/D converter 607 to various correction operationsor a compression operation.

A memory section 609 temporarily stores the image data, an external I/Fcircuit 610 communicates with an external computer and the like, and atiming generation section 611 outputs various timing signals to thesignal processing section 608 and the like. A whole control/arithmeticsection 612 conducts various computings and controls the whole camera,an I/F section 613 controls a recording medium, and a recording medium614 which is releasable such as a semiconductor memory records on orreads out from the recording medium. An external computer 615 is shown.

Next, an operation of the camera system in the present embodiment duringpicture taking will now be described. At first, the barrier 601 isopened, then a main power source is turned ON, subsequently a powersource of a control system is turned ON, and a power source of animaging circuit such as the A/D converter 607 is further turned ON.Subsequently, the whole control/arithmetic section 612 calculates aluminance of an object based on a signal which has been output from thelight measuring apparatus 605.

After the luminance of the object has been measured, real light exposurestarts. After the light exposure ends, the image signal which has beenoutput from the solid-state imaging apparatus 604 is converted fromanalog to digital form by the A/D converter 607, passes through thesignal processing section 608 and is written in the memory section 609by the whole control/arithmetic section 612. Afterwards, the dataaccumulated in the memory section 609 passes through the I/F section 613for controlling the recording medium while being controlled by the wholecontrol/arithmetic section 612, and is recorded in the releasablerecording medium 614. Alternatively, the data may pass through theexternal I/F section 610, and be directly input to the external computer615 or the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-209779, filed Aug. 18, 2008, which is hereby incorporated byreference herein in its entirety.

1. A photo-sensor comprising: a phototransistor for receiving light togenerate a photo-current and to output a current produced by amplifyingthe photo-current; a first MOSFET for detecting the current from thephototransistor; and a second MOSFET having a one non-controllingterminal connected to a base of the phototransistor, to discharge anexcessive photo-current in the phototransistor, wherein the secondMOSFET is turned OFF when a light quantity incident in thephototransistor is smaller than a predetermined value and a basepotential of the phototransistor is at a first potential, and the firstMOS FET operates in a saturation region under a condition that theincident light quantity of the phototransistor is larger than apredetermined value, and a gate potential of the second MOSFET is fixedso that the second MOS FET is turned ON, when the base potential of thephototransistor changes from the first potential to a second potentialduring a sensor operation period for outputting the photocurrent fromthe phototransistor.
 2. The photo-sensor according to claim 1, whereinthe second MOS FET further comprises the other non-controlling terminalcontrolled to be set at a potential between the first and secondpotentials.
 3. The photo-sensor according to claim 1, wherein the secondMOS FET is turned ON in a pixel reset period before the sensor operationperiod.
 4. The photo-sensor according to claim 1, wherein a referencecurrent for generating the gate potential of the second MOSFET and thepotential between the first and second potentials are generated by anidentical circuit as the circuit for generating a reference potentialfor generating a potential for controlling a current of a current sourceof the photo-sensor.
 5. The photo-sensor according to claim 1, whereinthe phototransistor is NPN type bipolar transistor, the first MOSFET isNMOS, and the second MOSFET is PMOS.
 6. The photo-sensor according toclaim 1, wherein the phototransistor is PNP type bipolar transistor, thefirst MOSFET is PMOS, and the second MOSFET is NMOS.
 7. A lightmeasuring apparatus comprising a photo-sensor according to claim
 1. 8. Acamera system comprising: a light measuring apparatus according to claim7; a solid-state imaging apparatus for obtaining an image signal of anobject through a lens focusing the object; and a signal processingcircuit for processing a signal from the light measuring apparatus andthe solid-state imaging apparatus.